This R01 application entitled Lead Optimization of Novel mGlu2 Negative Allosteric Modulators is in response to PAR-13-048 Drug Discovery for Nervous System Disorders. Major depressive disorder (MDD) is a serious disorder with an estimated lifetime prevalence of 19% and a societal cost >$83 billion a year in the USA. As with many other neuropsychiatric disorders, MDD is characterized by debilitating deficits in reward processing and dysfunctional neural circuitries mediating reward. Unfortunately, ~30-50% of patients suffer from treatment resistant depression (TRD) and do not respond to currently available antidepressants. Hence, there is a critical unmet need to develop effective therapeutics. Accumulating evidence suggests that the glutamatergic system may play an important role in the etiology, neurobiology, and potential management of reward processing deficits (RPDs). For example, group II metabotropic glutamate receptor (mGlu2 and mGlu3) antagonists have rapid and long-lasting antidepressant-like effects in multiple rodent tests of antidepressant activity. Antagonism of mGlu2/3 also increases neuronal proliferation in the hippocampus, an effect hypothesized to mediate antidepressant activity. However, it remains unclear whether group II mGlu antagonists may be effective for the management of RPDs that are resistant to current antidepressant medications. In the last decade, we have focused on developing small molecule group II mGlu allosteric modulators, which interact at less conserved allosteric sites topographically distinct from the mGlu orthosteric (glutamate) site. We have developed both lead and back-up series of mGlu2 negative allosteric modulators (NAMs) that are ready for full-scale chemistry optimization to provide compounds for suitable for behavioral studies. Thus, the overall objective of this project is to develop orally active mGlu2 NAMs for advanced in vivo proof-of-concept studies for the treatment of RPDs. Our Specific Aims are: 1) Design and synthesize optimized mGlu2 NAMs that are orally active in vivo; 2) Assess the potency and selectivity of mGlu2 NAMs in relevant in vitro assays; 3) Evaluate novel small molecule mGlu2 NAMs using in vitro ADME/T and in vivo pharmacokinetic (PK) assays; and 4) Characterize lead mGlu2 NAM probes in rodent in vivo models of antidepressant activity, anhedonia, and reward responsiveness. The mGlu2-selective NAMs generated will provide powerful tools for testing the role of mGlu2 in vivo. We are uniquely positioned to develop potent and selective small molecule mGlu2 NAMs with excellent PK properties for in vivo proof-of-concept studies, prior to the initiation of IND enabling studies. This multidisciplinary research program has the potential for significant scientific and medical impact by contributing to the discovery of new medications for neuropsychiatric disorders characterized by RPDs.